Sealed contact having tapered reed tips

ABSTRACT

A sealed contact includes a capsule enclosing a pair of magnetic reeds. Each reed has a fixed end sealed in the capsule and a flat movable portion with side edges substantially parallel to one another for a predetermined length and tapered toward one another from the end of the predetermined length to the tip of the movable portion. The reeds are positioned so that the movable portions overlap one another by an overlap length and define an overlap area which is less sensitive to variation in overlap length than a conventional sealed contact having reeds cut substantially normal to the parallel side edges.

BACKGROUND OF THE INVENTION

The invention is a sealed reed contact that is more particularly described as a contact having tapered tip reeds.

A sealed reed contact typically includes a capsule enclosing a pair of magnetic reeds. The reeds are sealed into the capsule so that one end of each reed is fixed, or immobile, and the other end of each reed is free to move. The movable ends of the reeds are positioned to overlap with one another so that they can be moved together for closing a circuit or moved away from each other to open the circuit. Either remanent or non-remanent magnetic material can be used for the reeds.

Because sealed reed contacts are used extensively in telephone switching systems, those contacts occupy a substantial portion of the space allotted for such a system. If the size of the sealed contacts were reduced, considerably space could be saved in all systems installations using a large number of the contacts.

To miniaturize reed contacts, it is desirable to reduce the length of the reeds while retaining the same reed stiffness. This can be accomplished by reducing the thickness of the reed and the diameter of the magnetic wire. Reduction of the diameter decreases the cross-sectional area of the reed, thereby reducing magnetic flux carried by the wire.

An important parameter of operating contacts is the magnetic force of attraction between the two reeds. This force is directly proportional to the square of the magnetic flux and is inversely proportional to the overlap area. In order to retain the closure force of a miniaturized contact at a magnitude equivalent to the magnitude of the larger prior art reed contacts, it is expedient to reduce the overlap area of the miniaturized contact and thereby offset the effect of the reduced flux.

Although reducing the overlap length can reduce the overlap area, the shorter overlap length causes substantial changes in magnetic attraction as a function of variations in the overlap length and misalignment of the reeds.

SUMMARY OF THE INVENTION

It is an object of the invention to miniaturize a magnetic reed contact.

It is another object to develop a magnetic reed contact that is less sensitive to variations of overlap length of the reeds.

It is also an object to develop a magnetic reed contact that is less sensitive to misalignment of the reeds.

These and other objects are realized in an illustrative embodiment of the invention wherein a sealed contact includes a capsule enclosing a pair of magnetic reeds. Each reed has a fixed end sealed in the capsule and a flat movable portion with side edges substantially parallel to one another for a predetermined length and tapered toward one another from the end of the predetermined length to the tip of the movable portion. The reeds are positioned so that the movable portions overlap one another by an overlap length and define an overlap area which is less sensitive to variation in overlap length than a conventional sealed contact having reeds cut perpendicular to the parallel side edges.

In the illustrative embodiment, tapered reed tips overlap one another by an overlap length defining an overlap area which is less sensitive to variation of overlap length than a conventional sealed contact.

It is another aspect of the illustrative embodiment to use a pair of reeds, each shaped so that the overlap area is less sensitive to misalignment of the reeds than a conventional sealed contact.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment of the invention will now be described by way of example, with reference to the attached drawing wherein:

FIGS. 1A and 1B show an arrangement of a conventional prior art sealed reed contact and a partial top view of the overlap area of the reeds;

FIG. 2A shows a sealed reed contact;

FIG. 2B shows an enlarged top view of the overlap area of the pair of reed contacts of FIG. 2A;

FIG. 3 is a comparative plot of the cumulative distributions of the percent variation of overlap area for FIGS. 1B and 2B; and

FIG. 4 is a comparative plot of the cumulative distributions of the percent variation of overlap area for FIGS. 1B and 2B when deviation from side to side is minimal.

DETAILED DESCRIPTION

Referring now to FIG. 1A, there is shown a sealed reed contact 10 including a capsule 12 enclosing a pair of magnetiic reeds 14 and 16. The reeds 14 and 16 are sealed into the capsule so that one end 17 of each reed is fixed and the other end 18 is free to move. The ends 18 are a part of flat movable portions having side edges which are substantially parallel to each other. Movable ends 18 overlap each other so that they can be moved together for closing a circuit or moved away from each other to open the circuit. Either remanent or non-remanent magnetic material can be used for the reeds.

FIG. 1B shows a top view of the reeds of FIG. 1A. The reeds 14 and 16 overlap by a length l₁ defining a cross-hatched overlap area A₁. It is noted that the movable ends 18 are both cut perpendicular, or normal, to the parallel side edges of the reed. Area A₁ is a function of the width w and the overlap length l₁ together with any change of overlap length, Δl, and any side to side deviation, or misalignment, δ .

    Area A.sub.1 =  (w-δ)(l.sub.1 -Δ l).           (1)

Nominal Area A₀ = w.sup.. l₁, when δ = 0 and Δl = 0. (2) ##EQU1##

Dimensions and variations resulting from manufacturing processes generally are known in the art. Illustratively, the nominal overlap area A₀ is selected to be equal to 1,500 mils², reed width w = 50 mils, and overlap length l₁ = 30 mils. The overlap length l₁ and the misalignment δ are subject to random variation resulting from manufacturing processes. Variance of the changes in overlap length, Δl, is approximately 9 mils, resulting in σ₃₁₃ l = 3 mils. Variance of misalignment δ is approximately 4 mils resulting in σ .sub.δ = 2 mils. It is believed that variance of Δl and δ are independent. A study of the variations of ΔA₁ /A₀, as a function of the distribution of probabilities of Δl and δ, follows.

First of all, there is shown a set of tables giving the probabilities for specific variations of Δl and δ. Probabilities P_(K) (Δl) = P(K-1<Δl<K) for variations of overlap length Δl, with variance σ.sub..sub.δl = 3 mils and the mean Δl = μ = 0, are as follows for normal functions:

When K = 1, ##EQU2## where Φ (Δl) is a normal function and dΔl is the differential of Δl. The equation (5) can be evaluated by reference to standard tables of values upon calculating ##EQU3## where N represents a normal function and c₁ and c₂ are integers from the integration boundaries of equation (5). ##EQU4## Probabilities P_(J) (δ) = P(J-1<δ<J) for variation of misalignment δ, with σ .sub.δ= 2 mils and the mean δ = μ = 0, are as follows for normal functions:

When J = 1 ##EQU5## where Φ(δ) is a normal function and dδ is the differential of δ. The equation (8) can be evaluated by reference to standard tables of values upon calculating equation (6). ##EQU6##

In the foregoing tables of probabilities, values of Δl and δ are representative values in mils of selected sample intervals used for calculating the probabilities of all possible coincident variations of Δl and δ. A summation of the resulting probabilities is equal to 1. ##EQU7##

For each coincidental combination of Δl and δ, there is a corresponding overlap area variation ΔA₁. The probability function of the variation of overlap area, P_(KJ) (ΔA₁ /A.sub. 0) is given by

    P.sub.KJ (ΔA.sub.1 /A.sub. 0) = P.sub.k (Δ l).sup.. P.sub.J (δ):                                                (11)

the total probability of occurrence of a particular sample interval of overlap area P_(I) (ΔA₁ /A₀) is determined by summing P_(KJ) (ΔA₁ /A₀) for all combinations of Δl and δ, wherein ΔA₁ /A₀ falls within the particular sample interval. The total probability for a particular sample interval of overlap area therefore is given by ##EQU8##

In equation (12) the subscript I designates each sample interval in percent change of area. This subscript integer is at the low end of the sample interval. Thus the probability P₀ (ΔA₁ /A₀) represents the probability for the sample interval between zero and 2 percent, and the probability P₂ (ΔA₁ /A₀) represents the sample interval between 2 percent and 4 percent.

Information for several discrete sample intervals and for a cumulative distribution function of the variation of overlap area is included in the following table. ##EQU9##

FIG. 3 includes a curve 31 showing the cumulative distribution of the variation of overlap area ΔA₁ /A₀ from the foregoing table. The cumulative distribution is plotted against the variation of area as a percent of nominal area shown as subscripts in the lefthand column of the foregoing table.

Referring now to FIG. 2A, there is shown another sealed reed contact 20 including a capsule 22 enclosing a pair of magnetic reeds 24 and 26. The reeds 24 and 26 are sealed into the capsule so that one end 27 of each reed is fixed and the other end 28 is free to move. Movable ends 28 overlap each other so that they can be moved together to close a circuit or moved away from one another to open the circuit. For each reed, the side edges of the movable ends 28 are substantially parallel to one another along a flattened portion for a length L. From the end of the length L to the tip of the reed, the sides taper toward one another to a point.

As shown in FIG. 2B, the reeds 24 and 26 are positioned so that the flattened movable portions overlap one another by an overlap length l₂ and are positioned so that each of the tapered sides of the two reeds intersects with a tapered side of the other reed. Although FIG. 2B is enlarged to show the cross-hatched overlap area in greater detail, overlap area A₂ is nominally equal to the overlap area A₁ of the squared-off, or normally cut, tip of the prior art arrangement shown in FIG. 1B and is less sensitive to variation in overlap length l₂ than the overlap area A₁ is sensitive to the variation of the overlap length l₁ of FIG. 1A.

For purposes of comparison of the illustrative embodiment, the width w in FIG. 2B is selected to be equal to 50 mils, the same as the width w in FIG. 1B. Variance of the side to side deviation δ for the embodiment of FIG. 2B is 4 mils with σ .sub.δ equal to 2, as in the embodiment of FIG. 1B.

Overlap length l₂ illustratively is selected to be 72.75 mils so that area A₂ nominally equals the area A₀ = 1500 mils². A distance y, which separates the ends of the parallel edges of the reeds, is selected to be slightly longer than three times anticipated standard deviation of the overlap length l₂. Since the variation of the overlap length is 9 mils with σ.sub..sub.δl = 3 mils, as in the embodiment of FIG. 1B, the distance y is chosen to be 10 mils.

Area A₂ is a function of the width w, the overlap length l₂, the distance y, an offset d, any change of the overlap length Δl, and any side to side deviation, or misalignment δ. ##EQU10## where tan α = y/d = l₂ /w.

For nominal area A₀ = 1500 mils², w = 50 mils, l₂ = 72.75 mils, y = 10 mils, δ = 0 and Δl = 0: d = 6.87 mils.

The corresponding total probability of occurrence of a particular sample interval of overlap area P_(I) (ΔA₂ /A₀) is determined by summing P_(KJ) (ΔA₂ /A₀) = P_(K) (Δl).sup.. P_(J) (δ) for all combinations of Δl and δ wherein ΔA₂ /A₀ falls within the particular sample interval. The total probability for a particular sample interval of overlap area therefore is given by ##EQU11##

As in equation (13), the subscript I designates each sample interval in percent change of area. Several sample intervals of information are given together with cumulative distribution information in the following table: ##EQU12##

The foregoing cumulative distribution of variation of overlap area A₂ /A₀ in percent and its presentation, as curve 32 in FIG. 3, represent the expected frequency of occurrence of variations of ΔA₂ /A₀ for the tapered tip reed contact, shown in FIG. 2B.

Curves 31 and 32 of FIG. 3 show that the area A₂ of FIG. 2B is less sensitive to variation of overlap length, Δl, and deviation, δ, than the overlap area A₁ of FIG. 1B is sensitive to variation of Δl and δ. There is a greater probability of lower percent variations of overlap area A₂ than of overlap area A₁ throughout the range of interest. In FIG. 3, all of the points where the curve 32 lies above the curve 31 are points at which the overlap area of the tapered tip is less sensitive to variation of overlap length Δl and of misalignment δ than the overlap area of the reeds cut normal to the side edges. The greater probability for lower variation of area shows that more of the possibilities have less variation of overlap area for the tapered tip configuration. This preponderance of lower variations of overlap area resulting from variations of overlap length and deviation provides switch contacts that have more uniform magnetic attraction between reeds during operation.

By extracting only the combinations of Δl and δ wherein δ is confined within the boundaries ± 0.5 mils, another cumulative distribution is compiled to show that the variation of overlap area of the embodiment of FIGS. 2A and 2B is less sensitive to variation of overlap length than the conventional squared-off tip of FIGS. 1A and 1B.

For each Δl, there is a corresponding ΔA₁ and ΔA₂. Their probability function has been given previously.

The total probability of overlap variation ΔA₁ /A₀ falling within a selected sample interval, P_(T) (ΔA₁ /A₀), is determined by summing P_(K) (ΔA₁ /A₀) for all Δl wherein the value of ΔA₁ /A₀ falls within the selected sample interval. ##EQU13## The subscript T designates each sample interval in percent change of area. The following table compiles some sample intervals. ##EQU14##

The total probability of overlap variation ΔA₂ /A₀ falling within a selected sample interval, P_(T) (ΔA₂ /A₀), is determined by summing P_(K) (ΔA₂ /A₀) for all Δl wherein the value of ΔA₂ /A₀ falls within the selected sample interval. ##EQU15## Values are compiled in the following table. ##EQU16##

FIG. 4 includes curves 41 and 42 showing respectively the cumulative distribution of the variation of overlap areas ΔA₁ /A₀ and ΔA₂ /A₀ from the foregoing tables. Curves 41 and 42 of FIG. 4 show that the area A₂ of FIG. 2B is less sensitive to variation of overlap length, Δl, than the overlap area A₁ of FIG. 1B is sensitive to variation of Δl. In FIG. 4, all of the points where the curve 42 lies above the curve 41 are points at which the overlap area of the tapered tip is less sensitive to variation of overlap length, Δl, than the overlap area of reeds cut normal to the side edges. This preponderance of lower variations of overlap area resulting from variations of overlap length provides switch contacts that have more nearly uniform magnetic attraction between reeds during operation.

The foregoing detailed description is illustrative of two embodiments of the invention and it is to be understood that additional embodiments thereof will be obvious to those skilled in the art. The embodiments described herein, together with those additional embodiments, are considered to be within the scope of the invention. 

What is claimed is:
 1. A magnetic reed contact comprising,a capsule, a pair of magnetic reeds, each reed having a fixed end sealed in the capsule and a flat movable end portion with side edges substantially parallel to one another for a predetermined length and tapered toward one another from the end of the predetermined length to the movable end, the reeds are positioned with respect to one another so that the movable ends overlap one another by an overlap length and define an overlap area, and the overlap area is less sensitive to variation in overlap length than the overlap area of a conventional reed contact having reeds cut substantially normal to the parallel side edges.
 2. A contact in accordance with claim 1 wherein the magnetic reeds are tapered to a point.
 3. A contact in accordance with claim 1 wherein the overlap area is less sensitive to misalignment of the reeds than the overlap area of a conventional reed contact having reeds cut substantially normal to the parallel side edges. 